Method of secretly marking a surface employing fission products

ABSTRACT

This invention relates to a method of secretly marking or placing intelligence on the surface of an article in such a manner that the markings are undetectable by ordinary methods. This is accomplished by embedding in the surface to be marked a small amount of recoil fission products from a spontaneously fissioning radioactive isotope. These fission products have a very low radioactivity, cause no radiation damage to the article so marked, are detectable only by very sensitive radiation detectors, and the intelligence is discernible only by a radioautographic means.

United States Patent Wing Apr. 16, 1974 METHOD OF SECRETLY MARKING A 2,969,307 1/1961 Fermi et al. 250 304 x SURFACE EMPLOYING FISSION 3,227,881 1/1966 Gordon 250/303 PRODUCTS James Wing, Rockville, Md.

Filed: Aug. 14, 1973 Appl. No.: 389,069

Related U.S. Application Data Continuation of Ser. No. 807,083, March 13, 1969.

Inventor:

U.S. CI 250/303, 250/304, 250/493 Int. Cl. G21h 5/02 Field of Search 250/271, 303, 304, 493

References Cited UNITED STATES PATENTS v 7/1952 De Ment 250/493 Primary ExaminerArchie R. Borchelt Attorney, Agent, or Firm-John A. Horan; James W. Weinberger; Robert James Fisher [5 7] ABSTRACT This invention relates to a method of secretly marking or placing intelligence on the surface of an article in such a manner that the markings are undetectable by ordinary methods. This is accomplished by embedding in the surface to be marked a small amount of recoil fission products from a spontaneously fissioning radioactive isotope. These fission products have a very low radioactivity, cause no radiation damage to the article so marked, are detectable only by very sensitive radiation detectors, and the intelligence is discernible only by a radioautographic means.

5 Claims, No Drawings METHOD OF SECRETLY MARKING A SURFACE EMPLOYING FISSION PRODUCTS CONTINUING APPLICATION This application is a continuation of application Ser. No. 807,083, filed Mar. 13, 1969.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION For years men have sought methods of communicating information to their friends and allies which, should the information fall into unfriendly hands, would prove unintelligible. Many of these methods are still in use today, such as the various methods of forming codes. These codes vary in complexity and the complexity of the code which is used depends upon the degree of security which is desired. I-lowever, codes can be broken,

. no matter how complex they are, permitting the intelligence to fall into undesirable hands.

A present-day problem is the protection of industrial equipment and personal property from those who would acquire it for their own use or for resale. Although much of this equipment is stamped with a serial number as an aid to identification, these numbers can be readily removed by those who are skilled in such art, rendering positive identification of one s own property tenuous at best.

I have developed a method which will solve both of the problems enumerated above. By my method, intelligence can be placed on a surface of a paper or similar object or a mark may be made on the surface of a piece of equipment which, even though they should fall into undesirable hands, are undetectable by any ordinary detection method. If the mark should be detected, it is unremovable from the surface short of destruction.

SUMMARY OF THE INVENTION My method consists of embedding in the surface to be marked radioactive fission products of a spontaneous fission radioactive isotope. These fission products contain a very low level of beta-radiation action and are only detectable to one with knowledge of their presence, by extremely sensitive radiation detectors.

It is therefore one object of my invention to provide a method of imparting a secret mark to a surface, which secret mark is detectable only by sensitive radiation detectors.

It is another object of my invention to provide a method for secretly marking the surface of articles, which marking is undetectable by other ordinary methods.

It is a further object of my invention to provide a method of secretly marking equipment which, even if detected, cannot be removed from the surface short of destruction of said surface.

Finally it is an object of my invention to'provide a method of marking surfaces with secret communicative intelligence, which intelligence is discernible only by radioautographic methods.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of this invention may be practiced by placing a spontaneous fissioning radioactive isotope close to the surface to be marked and allowing the isotope to remain in position for a period of time sufficient to deposit upon and embed beneath the surface of an adequate amount of radioative fission products to ensure their detection by appropriate means for a period of time after removal of the radioactive isotopic source, As used herein, the term fission products is used in the sense commonly used and accepted in the art to mean various isotopes of chemical elements produced by the fissioning of a heavy atom. The term fission product is intended to mean fission product nuclide where nuclide means a specie of atom characterized by the constitution of its nucleus, i.e., by the number of neutrons and protons it contains. The term fission product is not meant to include various secondary or subatomic particles which, although they may be produced in the fissioning of an atom, are not included in the commonly accepted definition of fission products.

A number of radioactive isotopes may be used as a source of the fission products. These sources may be either spontaneous fissioning sources having a convenient half-life, such as Cf Am Cm or GP, or it may be a source which can be induced to fission by bombardment with neutrons or other nuclear particles. A spontaneous fissioning source, such as CF which has a half-life of about 2% years, is suitable, as it is easily transportable, does not require an outside neutron source, and has a half-life which makes it convenient for use for a considerable period of time.

This method of marking may be used on any kind of nonsticky, fairly flat surface, including magnetic recording tape and paper. The'method can also be used on any sort of solid surface, such as metal.

The distance from the surface to be tagged to the radioactive source can vary from about 1.5 mm to about 4 mm. Smaller distances could cause an accidental contact with the surface with a resulting transfer of source material. Increased distances will result in increased exposure times, or lessened fission product transfer.

Only a small amount of source is necessary to tag the article as desired. Thus, about 1 microgram of Cf was found to be quite adequate.

The length of exposure of the surface to the spontaneous fissioning source is dependent upon the time for which detection is desired and is in direct proportion thereto. Thus, a collection time of 10 minutes from a l microgram Cf source at 1.5 mm distance is detectable by radioautographic means for up to 50 days. A one-hour collection time under the same conditions should be detectable for at least a 10-month period, while a one-minute exposure will last for 2 to 3 days.

The method of detection of the fission products is radioautography. By this method, a sheet of photographic film is placed upon the spot to be detected and exposed for a period of time before development. The length of time of exposure will necessarily depend upon the sensitivity of the film being used, the amount of fission products on the surface and the length of time since exposure. Types of film which have been found successful are Polaroid Type 47 film with an ASA rating of 3,000 and llford Industrial G X-ray film. Films with a high sensitivity are preferred to reduce the exposure time. Exposure times for Polaroid Type 47 film for the collection time described in the prior paragraph were 24 hours. Times other than the time given can readily be determined by one skilled in the art.

The fission product nuclides which result from the spontaneous fissioning of the radioactive source consist of a mixture of radioisotopes which are beta-ray emitters. Some of these isotopes, such as Br and C8140, have short half-lives, and some, such as Sr and Ce, have long half-lives. So, the radioactivities of the fission products collected will decrease at rates which become slower and slower with time. At some time, which depends on the amount of fission products collected, after the collection, the radioactivites of the fission products will be so low that they present no hazard to one in close contact with them and are undetectable by ordinary radioactive survey equipment over the normal background radiation. For example, after a 24-hour collection, the amount of radioactivity after a cooling period of 1 hour is 10 percent of the permissible level.

By the placement of a collimator containing a pattern between the source and the surface to be tagged, a pattern can be placed on the surface which will then be discernible on film exposed to the surface. This, by depositing a suitable dot-dash pattern or other pattern containing intelligence on the surface, ths invention becomes a method of secret communication, which can later be discerned only by one aware of its presence by radioautographic techniques.

An important advantage in the use of the method of this invention for secret communication is the built-in self-destroying feature. That is, the information is available for recovery by my detection method for only a specific period of time which is controlled by varying the exposure or collection time. After this period is expired, no intelligible detection of the information will ever be possible. Removal of the fission products from the surface of the article is very difficult, even with detergents, and organic solvents. Thus, once the article has been marked, it will remain marked, short of destroying the surface.

The following example is given as an illustration of the process of this invention and is not to betaken as limiting the scope or extent of the invention.

EXAMPLE exposed to the Cf source for periods of time of from 1 to 10 minutes. Radioautography of the foil was made with Polaroid Type 47 film having an ASA speed of 3,000. The exposure times varied from 0.5 to 24 hours. For l-minute and 10-minute collection exposures, film exposures made 1 and 14 days after the end of the collections gave clearly visible dot-dash patterns. The negatives showed a much better contrast than the positives. A visible pattern was obtained for the one-minute collection after a delay of 55 hours and a film exposure of 2.5 days.

It can be appreciated that the method of this invention has a great variety of useful purposes, such as marking items of equipment documents or even ransom money, for later positive identification and as a method of secret writing.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the inventionin which an exclusive property orprivilege is claimed are defined as follows:

l. A method of marking an article with a secrete mark comprising: positioning a small amount of a spontaneously fissioning isotope so as to not touch the surface of the article to be marked but close enough to the surface so that the article is within the range of the radioactive fission product nuclides emitted by said spontaneously fissioning isotope; maintaining said position for a period of time sufficient to allow emitted radioactive fission product nuclides to be transferred from the isotope of the article and thereby be embedded beneath the surface of the article; and removing said isotope.

2. The method in accordance with claim 1 wherein the article is marked with a secret marking containing communicative intelligence, which is discernible by radioautographic techniques, comprising: positioning a collimator containing a pattern between said spontaneously fissioning isotope and said article, and maintaining said collimator position until said isotope is removed.

3. The method of claim 2 wherein the spontaneously fissioning isotope is CF 4. The method of claim 3 wherein the spontaneously fissioning CF is placed from about 1.5 to 4 mm from the surface of the article to be marked, and the article is exposed to the CF for from 1 minute to 1 hour.

5. The method of claim 2 wherein the communicative intelligence is discerned byplacing photographic film over the area of the article wherein the fission products are embedded, allowing said film to remain in position for a period of time sufficient for said radioactive fission product nuclides to expose said film, and developing said exposed film. 

1. A method of marking an article with a secrete mark comprising: positioning a small amount of a spontaneously fissioning isotope so as to not touch the surface of the article to be marked but close enough to the surface so that the article is within the range of the radioactive fission product nuclides emitted by said spontaneously fissioning isotope; maintaining said position for a period of time sufficIent to allow emitted radioactive fission product nuclides to be transferred from the isotope of the article and thereby be embedded beneath the surface of the article; and removing said isotope.
 2. The method in accordance with claim 1 wherein the article is marked with a secret marking containing communicative intelligence, which is discernible by radioautographic techniques, comprising: positioning a collimator containing a pattern between said spontaneously fissioning isotope and said article, and maintaining said collimator position until said isotope is removed.
 3. The method of claim 2 wherein the spontaneously fissioning isotope is Cf252.
 4. The method of claim 3 wherein the spontaneously fissioning Cf252 is placed from about 1.5 to 4 mm from the surface of the article to be marked, and the article is exposed to the Cf252 for from 1 minute to 1 hour.
 5. The method of claim 2 wherein the communicative intelligence is discerned by placing photographic film over the area of the article wherein the fission products are embedded, allowing said film to remain in position for a period of time sufficient for said radioactive fission product nuclides to expose said film, and developing said exposed film. 